No, you are not to blame at all, cats are very good at keeping themselves warm if they need to. I once accidentally locked my cat out all night in the snow, was terrified in the morning, thought I had killed her! But she came back no worse for wear. It sounds as though it was just your cat's time to go.
If any old lady - cat, human, whatever, dies curled up in her sleep, it's the kind of end I'd want for myself! If she was curled up asleep she was happy and content. I'm glad you could offer dorothea a loving home <ex cat owner>
Dorothea is my grand daughters cat. Sweet cat came to live with me in the country. She was absolutely blossoming and seemed to have new life in her. Then one morning I found her dead, curled up on a chair in the living room. I feel guilty because the temperature had dripped that night and the livingroom gets coldest. I just wonder if the cold contributed to her death. That's the part that makes me so sad. I don't want to think she needed help to be warm and that I was not there for her.